1. Field of the Invention
The present invention relates to a belt driving apparatus, an image forming apparatus, a belt driving method, and a computer-readable medium, and more particularly to a belt driving apparatus using feedback signals for executing feedback control of a motor that drives the rotation of a belt, an image forming apparatus including the belt driving apparatus, a method for driving the belt, and a computer-readable medium on which a program is recorded for causing a computer to execute the belt driving method.
2. Description of the Related Art
In a conventional belt driving apparatus for driving, for example, an intermediary transfer belt of an image forming apparatus, a scale including plural indexes (e.g. adhesive marks) arranged in predetermined intervals along a peripheral direction of the belt is adhered onto a side edge part of the surface of the belt. The belt driving apparatus detects each index of the scale by using a sensor (reflection type photo sensor). Then, the belt driving apparatus detects the velocity of the belt based on the pulses output from the sensor in accordance with the detected indexes. Then, in accordance with the detected velocity of the belt, the belt driving apparatus controls a belt driving motor that drives the belt (feedback control). The feedback control enables the intermediary transfer belt to be driven at a desired velocity (uniform velocity).
Accordingly, the feedback control can prevent the velocity of the sheet conveyor belt or an intermediary transfer belt from changing and also prevent the positions of color images from deviating in a case where the belt is used, for example, in a color image forming apparatus (e.g. tandem type image forming apparatus) including plural photoconductors 91Y, 91C, 91M, and 91K (see FIGS. 10 and 11).
However, even where the scale is formed having indexes disposed at predetermined intervals along the peripheral direction of the intermediary transfer belt, factors such as anomaly in the length of the intermediary transfer belt may cause formation of a seam part between the starting end and the terminating end of the scale. Accordingly, in a case where the sensor detects the seam, the detection pulse corresponding to the detected seam (See FIG. 12) will have a longer pulse width compared to the pulse width of a detection pulse corresponding to a part other than the detected seam (See FIG. 13).
In a case where feedback control is performed using the detection pulse with a long pulse width, a control unit detecting the detection pulse with the long pulse width will determine that the belt velocity has decreased even though the actual belt velocity has not decreased. As a result, the belt driving apparatus will execute a feedback control process for increasing the velocity of the belt. This may lead to problems such as abnormal activation of the motor.
In order to solve this problem, Japanese Laid-Open Patent Application No. 2004-191845 discloses an image forming apparatus including a belt driving apparatus having an initial mark detecting sensor for detecting an initial mark provided on an endless belt so that the initial mark may be detected prior to reading marks provided on a scale of the endless belt. In a case where the initial mark is detected, the control by a first velocity controlling apparatus including a first scale sensor is cancelled and the control by a second velocity controlling apparatus including a second scale sensor is activated. Accordingly, the velocity of the endless belt is controlled by the second velocity controlling apparatus.
As another example, Japanese Laid-Open Patent Application No. 2004-198624 discloses an apparatus which detects changes of signals output from a sensor that detects the scale of an intermediary transfer belt. In a case where there is no change in the signals of the sensor, the apparatus determines that a seam of the scale is being detected and continues to drive the intermediary transfer belt at the same speed as the speed prior to the detection of the seam.
However, the image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 2004-191845 requires having the initial mark and the initial mark detecting sensor in addition to the first and second velocity controlling apparatuses. Furthermore, in order to increase the velocity of the belt driving motor to a predetermined velocity upon activation of the belt driving motor, the image forming apparatus requires complicated controls for increasing the velocity of the belt driving motor to the predetermined velocity while considering load characteristics of the motor. However, in a case where the initial mark is detected in the middle of increasing the velocity of the belt driving motor, the feedback signals (sensor signals) will change abruptly when control is switched from the first velocity controlling apparatus to the second velocity controlling apparatus in response to the detection of the initial mark. This is due to the phase difference between the signals output from the first sensor and the signals output from the second sensor. Since motors, in general, require a large amount of torque in a period between activation of the motor and reaching a predetermined driving speed, the motor is unable to respond to abrupt changes of feedback signals (sensor signals) during such periods. As a result, the motor may experience various malfunctions (e.g. out of step of the motor, discontinuation of the motor).
In the image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 2004-198624, controlling the velocity to be constant upon detection of the seam is not desirable during the period between activation of the motor and reaching a predetermined driving velocity. That is, even in a case when the seam of the scale is detected, it is desired to increase the velocity of the belt driving motor to the predetermined velocity without experiencing any malfunctions of the motor (e.g. out of step of the motor, discontinuation of the motor) during the period between activation of the motor and reaching the predetermined driving velocity.